Bonding of particulate high-surface-area noble metal oxide coatings to a proton-conducting solid polymer (ionomer) membrane for use in chlorine generators and oxygen gas generating devices are disclosed in Dempsey, et al., U.S. Pat. Nos. 4,224,121 and 4,331,569. The former describes a chlorine generating cell having anolyte and catholyte chambers separated by a solid polymer electrolyte in the form of a stable, selectively cation permeable, ion exchange membrane (such as is sold by duPont Corporation under the trademark Nafion(.RTM.)), and one or more catalytic electrodes including at least one thermally stabilized, reduced oxide of a platinum group metal bonded to the surface of the membrane. U.S. Pat. No. 4,331,569 discloses a gas generating apparatus and method which utilizes a novel noble metal oxide catalytic oxygen evolving electrode for such electrochemical systems as electrolysis cells and oxygen concentration cells. The electrochemical cells include a catalytic cathode and noble metal oxide catalytic anode positioned on opposite sides of, and in electrical contact with, an ion exchange membrane. In both of the above cited patents, there is an electrochemical generation of a gas from a liquid reactant.
Appleby in U.S. Pat. No. 4,610,938, discloses an improved electrochemical fuel cell wherein the catalytic surface of the electrode comprises a coating of fluorinated acid containing polymer. This device uses phosphoric acid as liquid electrolyte and generates power through the consumption of hydrogen and oxygen at a temperature of 190.degree. C.
Electrochemical capacitors of double-layer type are disclosed in prior U.S. Pats. including Nos. 3,536,963; 4,438,481; 4,630,176; 4,683,407; and 4,730,239. In general, the above patents describe double-layer capacitors which contain high-surface-area carbon structures as the two electrodes and liquid sulfuric acid as the electrolyte. The two electrodes are separated by a porous separator, which isolates the two electrodes electronically, but allows ionic conduction. To avoid the problems associated with the liquid electrolyte such as leakage and manufacturing problems, U.S. Pat. Nos. 4,638,407 and 4,730,239 describe the use of a polymer such as polyethylene glycol imbibed with an inorganic or organic salt to make it conductive as the electrolyte. However, the equivalent series resistance of such devices is large, which results in lower energy delivery and unacceptable discharge behavior for burst power applications.
The present invention avoids the problems associated with the liquid electrolyte by the use of a polymeric ionomer electrolyte. An electrode material is chosen that allows higher intrinsic capacitance through the participation of reversible surface and bulk reactions in addition to the double-layer capacitance. The capacitors described in the above mentioned U.S. Pats. use purely double-layer charge as the means of storing charge.
It is an object of the present invention to provide a high-energy-density all-solid ionomer electrochemical capacitor devoid of any electrolyte in liquid form by the use of solid polymeric perfluorocarbon ionomers. It is yet another object of the present invention to produce particulate-ionomer electrode structures in a membrane and electrode assembly (MEA) that provides a continuous proton-conducting ionic linkage throughout the structure. It is still another object of the present invention to produce a high-energy-density all-solid ionomer electrochemical capacitor in which there is no formation or consumption of either adsorbed or generated gases. It is still another object of the present invention to produce a particulateionomer electrode structure which is both ionically (transports protons) and electrically conductive (transports electrons) and is bonded to opposite sides of a solid proton-conducting ionomer membrane to form an integral all-solid ionomer configuration having unique energy storage and discharge properties on a short term basis (milliseconds to seconds).